I don't know how people have gotten the idea that Ayton is some kind of unathletic slug defensively. Physical tools will not be a problem. His lateral foot speed covering on the perimeter is as good or better than Jackson, Bagley, and Bamba. The issue with him is that he doesn't always put in the effort. He also has issues reading the play and making the correct rotation. His rim protection is a bigger concern for me than his ability to move his feet. He's terrible at holding position in the post which is hard to understand given how strong he looks otherwise.
Here's a super cut of some of his defensive highlights and lowlights:
I highly recommend watching the whole video but I want to point out the play at 11:44 which is a great example of where his problems are as a defender. His hands are down almost the entire play. He's actually in position to double team McKinley Wright (#25) under the basket but he doesn't use his length to deny the passing lane. The pass comes out to George King (#24) at the three point line and he immediately attacks the basket. Ayton rotates over but he's still got his hands down at his knees, waiting until the last possible second the go for the shotblock. This gives King a nice long look at Dallas Walton (#35) standing in the paint waiting for the rebound and an easy passing angle to get him the ball. After the pass Ayton appears to give up on the play. He's probably too far out of position to effectively contest at that point but it contributes to the perception that he's a lazy defender.
Most of this is coachable stuff -- keep your arms up! Make yourself an obstacle that players have to work to see around. Remember your primary job is to stay between the ball and the basket so don't over-commit on the perimeter and take yourself out of the play. There are players you see who make no decisions on defense. Those are the guys I can't stand. Ayton is making all kinds of decisions but unfortunately too many of them are the wrong ones. The physical tools are so good that I feel like his floor is as an average defender not a total liability. He should be able to hold his position better as he works on his lower body strength. Any NBA strength and conditioning team will get started on that right away. The mental aspect of processing the play faster is the key though and it's impossible to know if he will improve there or not. My feeling is if you get a coach who rides him from day one about keeping his hands up and making an effort to contest everything in his area he should become an effective defender. If he's allowed to continue his bad habits for a few years though it may be too late to unlearn them at that point.